1. Field of the Invention
This invention relates generally to portable communication devices such as cell phones, smart phones, and similar handheld devices such as tablet computers or tablets, and improvements thereto. In particular, this invention provides improvements in antennas and radio frequency (“RF”) shielding of such devices, by redirecting their RF radiation away from the body of a user while simultaneously maintaining the total radiated power (“TRP”) of such devices.
2. Background
Design requirements of mobile devices such as tablets, cellular phones, and smart phones place an ever increasing premium on the available space within these devices as their functions become more diverse. A device might function not only as a wireless telephone, but also as a music and video player; handheld computer; wireless device for browsing the web, retrieving email, and downloading or uploading files; still and video camera; GPS device and navigation system, and so forth. These functions and their related hardware bring with them greatly increased demands upon the antenna and generally require more radiated power for transmission, which might serve up to five frequency bands and, again, leave less space than ever before available for the antenna.
At the same time, although RF radiation has a necessary and beneficial role in wireless communications, its potential risk to human health have become a matter of concern. Cell phone antennae emit microwave energy (a type of RF radiation) when transmitting to cell towers. Consumers' heads and bodies absorb the majority of this energy, while the devices are proximate to them. The most intense radiation comes from RF waves nearest to the antenna, the radiation's source. In free space, this more intense “near field” energy would “propagate” to become far field energy, the form of RF signal that communicates with the tower. Mobile devices, including tablet computers, include manufacturers' warnings to keep the devices away from the head and body in order to avoid exposure to near field energy. Addressing this issue in the design of the antennae, and within the physical space constraints of mobile devices themselves, poses a particularly difficult technical challenge.
The Federal Communications Commission (“FCC”) limits the amount of radiation that portable communication devices (such as tablets and cellular telephones) may emit, as measured on the Specific Absorption Rate or “SAR” scale, in light of human health and safety concerns. Yet, as noted above, as wireless communications technology advances, devices have incorporated more data-intensive functions that require increasingly higher rates of data transfer between cell towers and the devices. While the functionality of mobile devices might, therefore, benefit from being able to increase the power output of their antennae, FCC regulations would not allow the resultant SAR increase.
In terms of its actual architecture, a “smart phone” (the catch-all nomenclature for devices like the iPhone and BlackBerry), for example, typically incorporates one or more internal antennae located at both the lower and upper parts of the phone and bordering the display area. The space for each antenna is usually limited to 1 cm times the width and thickness of the phone. The antenna is situated close to the back surface of the phone, on the side opposite to the user.
Tablet devices like the iPad have a larger surface area than a smart phone. The tablet can be configured as a Wi-Fi (WLAN) only device, and also as a Wi-Fi plus 3G mobile telephony capable device. Separate internal antennae would be provided for each type of wireless connectivity. The antennae are situated toward the back surface of the tablet, typically along a peripheral edge. Tablet models having 3G or other mobile telephony capability may have an elongated strip of plastic on the body above the location of the 3G or other telephony protocol antenna, to allow the mobile telephony signal to reach the internal antenna.
Some 3G-capable tablet computers include capacitive proximity sensors attached to the plastic strip, such that when the user's hand or body is in contact with the device at the location of the antenna, the TRP of the device is lowered in order to reduce the SAR experienced by the user. Lower TRP, however, reduces the quality of 3 G communication. Conventional external cases for such 3G-enabled tablet devices also cover and come into contact with the plastic strip having the attached proximity sensor, which may similarly result in reduced quality 3 G communications.